The invention relates to a halogen incandescent lamp for motor vehicles with an elongate bulb which is closed at one end and has a vacuumtight pinch seal at the other end, with at least one incandescent element arranged in the bulb, with a lamp cap connected to the pinch seal, and with current supply leads which are passed through the pinch seal to the incandescent element.
Such halogen incandescent lamps for motor vehicles are, for example, the familiar H4 or H7 car lamps. The bulb is usually made of quartz or hard glass and has a substantially cylindrical shape. The diameter of the bulb, however, need not necessarily be constant along its length. The tubular bulb is closed at one end in a fusion process. Depending on the type, one (H7) or two (H4) incandescent elements, usually made of coiled tungsten wire, is or are arranged in the bulb. The incandescent elements are usually connected to the current supply leads by means of welding, said leads being formed, for example, from molybdenum wires and being introduced through the still open end of the bulb. The bulb is evacuated, provided with a suitable filling, and closed vacuumtight by means of a pinch seal. In this pinch seal, the current supply leads are fixed, so that the incandescent elements occupy a defined position in the bulb. The position may be adjusted during the pinching process. The lamp cap, usually made of several metal parts (or alternatively synthetic resin parts), which are welded/fused together, is fixedly connected to the pinch seal of the bulb. At least one part is constructed here such that it can serve as a reference for the position of the incandescent elements in the bulb. When incorporated in a headlamp assembly of a motor vehicle, this reference surface is used for placing the incandescent element(s) of the lamp in a defined position in the reflector of the headlamp assembly. As a result of this, a reflector of mating design is capable of generating a suitable light beam from the incandescent element.
A halogen motorcar incandescent lamp is known from German utility model G 86 00 640 U1. The lamp described therein comprises a glass bulb in which an incandescent body is arranged in a gas comprising halogen, which lamp bulb has a seal at a first end through which the current supply conductors extend to the incandescent body. The glass bulb is provided with a light-absorbing layer on its outer surface at the other end. A yellow interference filter of alternate layers of comparatively high and comparatively low refractive index is provided on the outer surface of the glass bulb. The lamp bulb further comprises a light-absorbing layer at the first end of the bulb. Undesirable reflections on separate yellow outer bulbs are prevented by means of the yellow interference filter on the glass bulb. The application of the yellow interference filter on the glass bulb of the lamp means that yellow glass need not be used for the outer bulb. The yellow interference filter is provided on the entire glass bulb, so that the lamp generates yellow light, in particular for reducing reflections in front of a motor vehicle head light, for example in the case of fog. The light-absorbing layer at the first, pinch-sealed end of the lamp bulb also shown as part of the halogen motorcar incandescent lamp counteracts the radiation of blue light. The radiation in a nonessential location of a portion of the light generated by the incandescent body is prevented by means of an absorption filter. The two layers acting as an optical filter cause the halogen incandescent lamp to generate a yellow light beam. Known lamps have the disadvantage that the coating of the entire bulb leads to the subjective impression that the generated light is darker than in the case of colorless glass bulbs.
It is an object of the invention to provide a halogen incandescent lamp for motor vehicles which offers improved illumination properties when used as a light source in a motor vehicle headlamp.
This object is achieved by means of a halogen incandescent lamp for motor vehicles with the characteristics as defined in the first claim. Although it is favorable to use quartz or hard glass, it is obvious that any transparent material may be used for the bulb without departing from the scope of protection of the claim. The annular shape of the optical filter results from the shape on the substantially cylindrical bulb. Nevertheless, the annular optical filter may also extend over the closed end of the bulb, so that a cap with a ring is created on the bulb. The width of the ring on the bulb may be determined through taking into account the application and other components such as, for example, the reflector used. Just as the width of the annular optical filter may vary in dependence on the requirements, the amount of the total light output reduced by the optical filter may also be adapted (for example to legal requirements). This may be achieved through the choice of a suitable material or the application of several layers. Known processes (dipping, spraying, vapor deposition) may be used for providing such an optical filter.
This renders it possible in a simple manner both to change the subjective impression of the generated light and to improve the objective properties through emphasizing a desired region of the spectrum. Since a major portion of the bulb remains clear, a high proportion of white light is obtained in principle. A particularly interesting advantage of the lamp according to the invention is the possibility of increasing the total light output through optimization of the incandescent elements and the filling in the bulb and to adapt it to legal requirements by means of the annular filter. It is possible for this purpose, for example, to effect a color shift to short-wave portions through an increase in the reflectivity. Furthermore, the light issuing from the clear region of the bulb is particularly suitable for use in forming a light beam in a reflector. Since more light can be projected onto the road by means of the reflector, the lamps according to the invention are eminently suitable as light sources in motor vehicle headlights.
In a preferred embodiment of the invention, an absorption filter is provided for forming the annular optical filter, which is advantageously formed by a coating of at least one layer of a colored metal oxide. Absorption filters are particularly suitable because they generate no reflected light which may issue from the bulb in an uncontrolled manner. Furthermore, the metal oxides may be provided on the lamp in a simple manner in dissolved form by known methods. Various materials (for example, cobalt aluminate CoAl2O4, neodymium) are known, so that a suitably adapted absorption filter may be chosen. In particular, a wavelength-dependent absorption may be adjusted. In a further development of the invention, therefore, it is provided that the absorption filter absorbs mainly light with wavelengths above 500 nm. Since the non-coated portion of the bulb is clear, light with a high blue component is achieved in this manner. This has the advantage in particular that persons and objects (for example traffic signs, direction posts, informative placards) irradiated on the road reflect better. Safety is clearly enhanced through a greater emphasis on the higher-frequency region of the spectrum. Nevertheless, however, the emphasis on a different region of the spectrum may also be useful for certain applications. A suitable absorption filter may be provided for all cases through the use of a corresponding material within the scope of the invention on which the lamp is based.
In a further embodiment of the lamp according to the invention, a reflection filter is provided for forming the annular optical filter. Such a reflection filter is preferably an interference filter with alternate layers of high and low refractive index.
In a further embodiment of the invention, the bulb has a light-absorbing layer at its closed end, and the annular optical filter extends over and beyond the light-absorbing layer onto the bulb. Various motor vehicle lamps (for example H4) have a cap which is impermeable to light at the closed end facing away from the reflector in the headlamp. The annular optical filter according to the invention may be provided also in the case of such lamps. The optical filter may be provided on the layer impermeable to light or between the bulb and the layer impermeable to light.
In an advantageous embodiment of the invention, the annular optical filter is provided from the closed end of the bulb up to that level on the bulb where a first incandescent element is positioned. The width of the annular filter thus extends from the end of the bulb up to the level on the bulb where approximately the incandescent element or, in the case of several incandescent elements, the first element arranged in longitudinal direction in the bulb starts. The filter may end abruptly at the level of the incandescent coil or, for example, may become gradually fainter. If an optional light-absorbing layer was additionally provided on the closed end of the bulb, the annular optical filter will extend from the end of this cap up to the position of the first incandescent element.